1. Field of the Invention
The present invention relates to a nitride-type compound semiconductor laser device capable of emitting light from a blue range to a UV range, and further relates to a laser apparatus incorporating the same.
2. Description of the Related Art
Gallium nitride is a III-V group compound semiconductor material whose bandgap is as large as about 3.4 eV, and has been positively studied as a material to be used in a light emitting device capable of emitting light from a blue range to a UV range.
Referring to FIG. 8, a conventional gallium nitride-type compound semiconductor laser 40 will be described.
The semiconductor laser 40 of FIG. 8 includes a layered structure in which an n-type GaN buffer layer 22b, an n-type AlGaN cladding layer 23, an As-added GaN light emitting layer 54, a p-type AlGaN cladding layer 28 and a p-type GaN contact layer 59 are formed in this order on a 3C-SiC substrate 46. An n-type GaN current blocking layer 45 including a stripe-shaped opening is formed on the p-type AlGaN cladding layer 28. A p-electrode 10 and an n-electrode 11 are provided on the upper surface of the p-type GaN contact layer 59 and on the bottom surface of the 3C-SiC substrate 46, respectively. A current flowing from the p-electrode 10 to the n-electrode 11 is constricted by the n-type GaN current blocking layer 45. Such a gallium nitride-type compound semiconductor laser is disclosed in, for example, Japanese Laid-open Publication No. 7-249820.
However, it is not practical to form electrodes over the entire upper and bottom surfaces of the device, as in such a conventional gallium nitride-type compound semiconductor laser 40 illustrated in FIG. 8. Thus, in many cases, there are provided areas 52 along sides of the device which are not covered by the electrodes, as shown in FIG. 9.
FIG. 10 is a perspective view illustrating the conventional gallium nitride-type compound semiconductor laser 50 of FIG. 9 having the areas 52, as it is incorporated into a package. In FIG. 10, reference numeral 51 denotes a photodiode and reference numeral 53 denotes a back surface of the semiconductor laser device 50 which corresponds to the surface on which the electrode 10 is formed in FIG. 9. FIGS. 11A and 11B are schematic diagrams each illustrating the positional relationship between the conventional gallium nitride-type compound semiconductor laser 50 and the photodiode 51 incorporated in the same package illustrated in FIG. 10. Specifically, in FIGS. 11A and 11B, the photodiode 51 is viewed from directions indicated by arrows A and B in FIG. 10, respectively.
In many cases, the photodiode 51 is provided on the side of the back surface 53 of the semiconductor laser 50, as illustrated in FIG. 10, for detecting the output from the back of the semiconductor laser 50 and for controlling the semiconductor laser 50 based on the detected output. A layered structure constituting such a conventional gallium nitride-type compound semiconductor laser is transparent to the emission wavelength. Therefore, as illustrated in FIGS. 11A and 11B, the light generated in the active layer may partially leak out of the device through the areas 52 which are not covered by the electrode 10 and be incident upon the photodiode 51 as noise, thereby causing an erroneous control.
Moreover, since such a package typically includes a cap welded thereto for airtight sealing, the leaked light impinges on the inner wall of the cap and causes multiple reflection. Thus, even when the photodiode 51 is provided at a position other than on the side of the back surface 53 of the semiconductor laser 50 as illustrated in FIG. 10, such leaked light may still be incident upon the photodiode 51 as noise, thereby causing an erroneous control.
A nitride-type compound semiconductor laser device of the present invention includes a substrate and a layered structure provided on the substrate. A light absorption layer having a bandgap smaller than a bandgap of an active layer in the layered structure is provided in a position between the cladding layer, which is provided on a side opposite to a mount surface with respect to the active layer, and a surface of the layered structure which is opposite to the mount surface.
The light absorption layer may include a stripe-shaped opening; and an opaque electrode may be provided on the surface of the layered structure which is opposite to the mount surface so as to completely block a light path which is travelling from a light emitting section through the stripe-shaped opening to the surface of the layered structure which is opposite to the mount surface.
The laser device may further include another light absorption layer having a bandgap smaller than the bandgap of the active layer, which is provided on the same side as the mount surface with respect to the active layer.
The light absorption layer may be formed of GasAltIn1xe2x88x92sxe2x88x92tN (0 less than s less than 1, 0xe2x89xa6t less than 1 and 0 less than s+t less than 1).
A laser apparatus of the present invention includes a nitride-type compound semiconductor laser device as set forth above and a light receiving device provided in a same package.
In the nitride-type compound semiconductor laser device of the present invention, when a voltage is applied across the electrodes, a current flows through the semiconductor layered structure from the p-electrode to the n-electrode, thereby generating light in the active layer. The generated light partially seeps into the cladding layer. However, since the light absorption layer which has a bandgap smaller than that of the active layer is provided on the side opposite to the mount surface for a sub-mount or the like with respect to the active layer, the light seeping into the cladding layer is either absorbed by the light absorption layer, or blocked by the upper electrode after passing through the opening of the light absorption layer which provides a current path. Therefore, even if a photodiode is provided in the same package with this gallium nitride-type compound semiconductor laser, such light is never incident upon the photodiode as noise to cause an erroneous control.
More preferably, another light absorption layer which also has a bandgap smaller than that of the active layer may be provided on the same side as the mount surface for a sub-mount or the like with respect to the active layer. In such a case, even when the substrate material is transparent to the emission wavelength, the light seeping into the cladding layer is never reflected by the mount surface, and thus noise is not generated.
Still more preferably, the light absorption layer may have a conductivity type opposite to that of the layers provided on the same side as the light absorption layer with respect to the active layer, while a stripe-shaped opening may be provided in the light absorption layer by removing a portion thereof. In such a case, the light absorption layer can exhibit a current blocking function.
Thus, the invention described herein makes possible the advantages of: (1) providing a nitride-type compound semiconductor laser device without light leakage which may cause an erroneous control of a photodiode to be provided in the same package with the nitride-type compound semiconductor laser device; (2) providing a laser apparatus in which the aforementioned nitride-type compound semiconductor laser device and a light receiving device are provided in the same package; and (3) providing a circuit for photoelectrically converting an output of the above nitride-type compound semiconductor laser device by the light receiving device and for controlling a current to be input to the nitride-type compound semiconductor laser device based on the photoelectrically converted output.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.